Optical devices uncover how flies walk upside down

In a series of experiments aimed at learning more about how flies can walk upside down, researchers
at Max Planck Institute have determined that flies place their feet very carefully
to keep them stuck to the ceiling. The work is part of an overall effort to better
understand the mechanism involved, and it could be useful in applying it to high-tech
robots.

Since the 19th century, scientists have been studying
how insects adhere to a ceiling. Various insects have different systems for staying
put. Typically, they either have pads on their feet covered by tiny hairs that increase
the contact area with the substrate or they have very soft, adaptable pads. In addition,
many insects, such as houseflies, secrete fluid onto the pads of their feet to enhance
capillary forces in the contact.

Stanislav N. Gorb of the Evolutionary
Biomaterials Group at Max Planck Institute for Metals Research in Stuttgart, Germany,
has used a variety of optical instruments to study how flies walk. Three years ago,
using high-speed video, the group monitored the flies’ movements.

Researchers at Max Planck Institute for Metals Research used fiber
optics to measure the forces exerted when a fly walks upside down. They observed
that feet of a common bluebottle fly has thousands of tiny hairs that help it walk
on inverted surfaces. Courtesy of Stanislav N. Gorb.
The video showed that flies move alternate,
opposite legs on each side when walking upright. This keeps three legs in place
while three move. Upside down, they move only two legs at a time so that more feet
remain in place to counteract gravity. The high-speed camera, from Photron Inc.
of San Diego, was especially useful in watching the flies lift their feet. Because
the attachment between the footpads and the surface is so strong, the insects have
to use four motions to free their feet: shifting, twisting, rotating and pulling.

Recently, Gorb and his colleagues designed
an experiment using a glass spring equipped with fiber optic sensors from Tetra
GmbH of Ilmenau, Germany, to measure the contact forces as flies walked across the
substrate. They presented the work at the annual meeting of the Society for Experimental
Biology in Canterbury, UK.

The fiber optic sensors measured deflections
in two directions. “As soon as the fly touched the glass platform attached
to the spring, we measured the spring deflections, which were recalculated into
forces,” Gorb said.

The scientists found that flies do
not press, or only very gently press, on the ceiling; they simply slide their footpads
along it. They also discovered that the angle at which a fly’s foot touches
the surface helps control the attachment forces. Gorb said that changing the angle
from roughly 30° to 90° can mean an eightfold variance in attachment strength.

The researchers are developing materials
that can mimic the adhesive forces of fly feet, in hopes of creating robots or devices
that could assist climbers. From here, Gorb said, they plan to replicate the experiments
with other insects.